1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus for transmitting control information in a Wireless Local Area Network (WLAN) system supporting multiple antennas.
2. Description of the Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a WLAN is a technique which wirelessly enables access to the Internet at home or companies or in a specific service providing area using mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), on the basis of radio frequency technology.
Since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization of WLAN technology) has been set up February, 1980, lots of standardization task are being performed.
The initial WLAN technology was able to support the rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication using a 2.4 GHz frequency band in accordance with IEEE 802.11, but recently can support the maximum rate of 54 Mbps using Orthogonal Frequency Division Multiplex (OFDM). In addition, in IEEE 802.11, the standardization of various techniques, such as the improvement of Quality for Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is being put to practical use being developed.
IEEE 802.11b from the IEEE 802.11 supports a maximum transmission speed of 11 Mbs while using the 2.4 GHz frequency band. IEEE 802.11a commercialized since the IEEE 802.11b has reduced the influence of interference as compared with the very complicated 2.4 GHz frequency band by using a 5 GHz frequency band not the 2.4 GHz frequency band and also improved the transmission speed up to a maximum of 54 Mbps using the OFDM technique. However, the IEEE 802.11a is disadvantageous in that the transmission distance is shorter than that of the IEEE 802.11b. Further, IEEE 802.11g implements a maximum transmission speed of 54 Mbps using the 2.4 GHz frequency band like the IEEE 802.11b, and it is significantly being in the spotlight because it satisfies backward compatibility. The IEEE 802.11g is superior to the IEEE 802.11a even in the transmission distance.
Further, as a technique for overcoming the limit to the transmission speed pointed out as vulnerabilities in the WLAN, there is IEEE 802.11n which has recently been standardized. The IEEE 802.11n has its object to increase the speed and reliability of a network and to expand the operating distance of a wireless network. More particularly, the IEEE 802.11n is configured to support a High Throughput (HT) having a data processing speed of a maximum of 540 Mbps or more and based on a Multiple Inputs and Multiple Outputs (MIMO) technique using multiple antennas on both sides of a transmitter and a receiver in order to minimize transmission error and optimize the data rate. Further, the IEEE 802.11n may use a coding method of transmitting several redundant copies in order to increase the reliability of data and OFDM (Orthogonal Frequency Division Multiplex) in order to increase the speed.
With the wide spread of the WLAN and various applications using the WLAN, a necessity for a new WLAN system for supporting a higher throughput than the data processing speed supported by the IEEE 802.11n is recently gathering strength. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been newly proposed in order to support the data processing speed of 1 Gbps or more. The name of the VHT WLAN system is arbitrary, and a feasibility test for a system using 4×4 MIMO and a channel bandwidth of 80 MHz or more in order to provide the throughput of 1 Gbps or more is being performed.
The VHT WLAN system now being discussed includes two kinds of methods using a frequency band of 6 GHz or less and a frequency band of 60 GHz. If the frequency band of 6 GHz or less is used, a possibility of coexistence with conventional WLAN systems using the frequency band of 6 GHz or less can become problematic.
Meanwhile, the physical (PHY) layer architecture of the IEEE 802.11 consists of a PHY Layer Management Entity (PLME), a Physical Layer Convergence Procedure (PLCP) sublayer, and a Physical Medium Dependent (PMD) sublayer. The PLME functions to manage the physical layer while cooperating with a MAC Layer Management Entity (MLME). The PLCP sublayer functions to transfer a MAC Protocol Data Unit (MPDU), received from the MAC layer, to the PMD sublayer or transfers frames, received from the PMD sublayer, to the MAC layer between the MAC layer and the PMD layer in accordance with an instruction of the MAC layer. The PMD sublayer is a lower layer of the PLCP and it enables the transmission and reception of a physical layer entity between two stations through a radio medium.
The PLCP sublayer attaches additional fields, including information necessary for a physical layer transceiver, to an MPDU in a process of receiving the MPDU from the MAC layer and sending the MPDU to the PMD sublayer. The fields attached in this case can include a PLCP preamble for the MPDU, a PLCP header, tail bits over a data field, and so on. The PLCP preamble functions to have a receiver prepare for a synchronization function and antenna diversity before a PSDU (PLCP Service Data Unit=MPDU) is transmitted. The PLCP header includes information about a frame (e.g., PSDU Length Word (PLW)), information about the data rate of a PSDU portion, and information about header error check.
The PLCP sublayer generates a PLCP Protocol Data Unit (PPDU) by adding the above fields to the MPDU and sends the PPDU to a reception station via a PMD sublayer. The reception station restores data by acquiring the PLCP preamble of the received PPDU and information about data restoration from the PLCP header.
In case where a variety of legacy stations and VHT stations, such as IEEE 802.11 a/b/g/n, coexist, the legacy station cannot recognize or erroneously recognize the PLCP format and thus can malfunction. In order to prevent the above problem, in case where the PLCP format recognizable by the legacy stations and a format for the VHT stations are attached to all transmission data so that the formats can be recognized by all the stations, overhead is increased, thus hindering the efficient use of radio resources. Further, in a WLAN system supporting Multi-User (MU)-MIMO, in case where radio frames are spatially multiplexed for multiple users and transmitted, there is a problem that a station (i.e., not a target of transmission) cannot recognize the radio frames. It is also expected that the amount of control information necessary to send, receive, and decode data will be increased according to the MU-MIMO support.
Consideration is required for a new frame format for a method of transmitting control information in a WLAN system supporting MU-MIMO and for a VHT WLAN system which can accommodate increasing control information, support backward compatibility, and guarantee coexistence with a legacy station.